1. Field of the Invention
This invention relates to a microcomputer having a programmable read only memory, and more particularly to a microcomputer for checking a number of writes to the programmable read only memory (PROM).
2. Description of the Prior Art
For example, it will be observed from FIG. 3 that a conventional microcomputer has a programmable read only memory.
Referring to the drawings, PROM(2), RAM(3), PIO(4) and SIO(5) are interconnected to a central processing unit (CPU)(1) by a control bus A, an address bus B and a data bus C, then all of them are integrated in a package.
As to the above-mentioned one chip microcomputer, a user is able to write into the PROM and use it as suits its own purpose.
Generally speaking, in the case of writing into a PROM, the PROM is provided with a high voltage, higher than the usual operating voltage, and the PROM is broken down after a number of writes to a memory location. It is therefore necessary for a broken down PROM to be exchanged for a new one if the number of writing times in the PROM exceeds the limit.
However, with respect to a conventional microcomputer, the number of writes to the PROM is not recognized by any user as a significant event, malfunctioning of the microcomputer is caused by being overworked after the PROM is broken down.